The invention relates to a method of removing alkali halides in a washing step from the reaction products of xylylene dichlorides and/or xylylene dibromides with bisphenols in the presence of alkaline compounds or alkali salts, at 85.degree. to 160.degree. C. and, in some cases, under addition of end-group blocking monophenols or monohalogen alkyl compounds, in the presence of ketones exhibiting a miscibility gap with water, followed by the addition of water or a mixture of ketones and water to the reaction mixture and then filtration thereof with recycling of the filtrates.
The reaction as above, for the production of oligomeric, brominous xylylene bisphenol ethers, is performed advantageously in the presence of ketones as solvents. Corresponding amounts of alkali halides occur as byproducts (cf. DE-OS No. 2,929,914 corresponding to copending application U.S. Pat. No. 06/167,555 of 07/11/80) and now U.S. Pat. No. 4,371,709.
The process is especially suitable for the oligomeric brominous xylylene glycol bisphenol ethers which bear as terminal groups aryloxy moieties formed from phenols, especially brominated aryloxy moieties. These ethers have the formula: ##STR1## wherein p amounts to 2 to 10 and each X is Br, Cl or H, n=0 to 4 and each Y is Br, Cl or H, at least two Br being contained in the sum of X+Y, and Z is a direct bond, --CH.sub.2 --, --C(CH.sub.3).sub.2 -- or --SO.sub.2 --. The ethers are useful as fire-retardants in plastics.
Phenols forming suitable terminal groups are especially tribromophenol, in some cases also pentabromophenol and the corresponding chlorophenols.
The bromine content of these ethers is to be not less than 40 weight percent, bromine contents above 60 weight percent being preferred.
The preparation of the oligomeric, brominous xylylene bisphenol ethers is accomplished in a conventional manner by dissolving the halogenated bisphenol with a stoichiometric to slightly less than stoichiometric amount of alkali hydroxide (as aqueous solution) in the ketone solvent, bringing the solution to reaction in the autoclave for 1 to 4 hours with a more than stoichiometric amount of halogen xylylene dihalide, then adding the necessary amount of a halogen monophenol alkali salt to close the terminal group, and letting the reaction complete itself for another 1 to 4 hours.
To obtain a pure end product, the alkali halides must be removed as completely as possible from the reaction product. This is accomplished by washing and filtration, resulting in large amounts of waste waters when the known process is followed. The ketones used as solvents and the alkali halides that are formed end up in some washing filtrate fractions in such low concentrations that the complete recovery and recycling of these substances is difficult and costly. However they occur in high enough concentrations so that they must be removed to avoid environmental pollution. Removal is therefore accomplished inspite of the costs.
The problem for operation on an industrial scale, was to find a method in which practically complete recovery of the ketones and alkali halides is accomplished and the reuse of the ketones made possible, within reasonable energy consumption restraints.
The invention provides an effective method of avoiding prior art difficulties by requiring that a ketonous and an aqueous phase after separation of the filtrate into two liquid phases are recovered by separate evaporation of the two phases, both of the condensates are combined to a mixture, the ketone-water mixture is reused as washing media in the washing process, the filter cake, formed in a process of the kind described above, is stirred in the recycled ketone-water mixture, and is filtered a second time.
It is economically advantageous to process the phases which form when the filtrate fractions after allowing to separate into one phase which is predominantly aqueous and one which is predominantly ketonous by separate evaporation and to reuse both of the condensates in the washing process as washing media; the condensate of the ketonous phase being further usable in the reaction as a solvent.
In the proposed process normally about 20% or less of the amount of water required by the prior method is required for washing out the alkali halide.
Furthermore, because there is relatively high concentration of alkali halide in the filtrate, the mixture separates still more easily into a less aqueous ketone phase and into a less ketonous aqueous phase than has been observed in the examples of DE-OS No. 2,929,914.
It has been found that the simple evaporation of the ketonous filtrate phase results in a ketone condensate having low water content and which can be reused entirely or at least in part as solvent in the reaction and/or as a washing medium in the washing process. The complete separation of the ketone and water, which is very difficult to accomplish, is not necessary for this reuse. The aqueous filtrate phase is separated by simple evaporation into a ketonous water condensate which can be reused as a washing medium in the washing process and in the crystaline alkali halogenide. Suitable ketones are the aliphatic and cyclic ketones of 3 to 10 carbon atoms. Examples of such ketones are: acetone, methyl ethyl ketone, methyl isopropyl-, methyl isobutyl-,methyl tert.-butyl-,diisobutyl-,and di-tert.-butyl-ketone, 2-pentanone, 3-pentanone, 2-hexanone, 3-hexanone, heptanone, cycloheptanone, cyclohexanone, acetophenone, isophorone, but preferably 2-butanone, methyl isobutyl ketone or cyclohexanone, of which 2-butanone is greatly preferred.
After the reaction there will remain, for example, a reaction mixture containing 84 wt.-% of polytetrabromo-p-xylylene-3,3',5,5'-tetrabromodianether (hereinafter called TBX-TBD), including byproducts, and 16 wt.-% of NaBr, in a mixture of methyl ethyl ketone and water.
The prior art method of processing the reaction mixture at this point was to add water, the amount of water in the reaction mixture being increased to such an extent that the sodium bromide could go completely into solution. The reaction mixture is then filtered in a paring-type centrifugal separator and washed. To assure that the residual sodium bromide concentration will be less than 0.4% by weight in the dried end product, the washing must be performed with a total of 4.5 to 5 kg of water per kg of TBX-TBD.
After the filtration, a methylethyl ketone and water mixture remains, which separates into a methyl ethyl ketone phase and an aqueous phase. Because of the high concentration of the methyl ethyl ketone, the methyl ethyl ketone can be recovered from the methyl ethyl ketone phase for reuse. The high sodium bromide concentration also permits its recovery from the aqueous phase.
After the washing there remains a wash water containing 1.5 wt.-% of sodium bromide and about 0.5 wt.-% of methyl ethyl ketone. These amounts of sodium bromide and methyl ethyl ketone, absolutely considered, are still relatively great although economical recovery is not possible on account of their low concentration. However, it is necessary, in order to avoid pollution of the environment from discharged waste water, to remove the ketone and halide from the waste water.